The Illusion of Depth: How Ray Casting Birthed the Modern Shooter

In the early 1990s, PC gaming was facing a significant technological hurdle. Developers dreamed of creating immersive, fast-paced 3D worlds, but the home computer hardware of the era was woefully unequipped for the task. The dominant method for creating 3D graphics—rendering environments out of mathematical shapes called polygons—was computationally expensive. While flight simulators like Red Baron managed it, the action was inherently slow, and the environments were stark and untextured. For a high-speed, action-packed game running indoors, true 3D was a non-starter.

Then came a brilliant workaround that changed the trajectory of the gaming industry: ray casting. It was not true 3D, but a clever, highly optimized illusion that allowed games like Wolfenstein 3D to run at blistering speeds on average PCs, effectively giving birth to the first-person shooter (FPS) genre.

The Polygon Problem

To understand the genius of ray casting, one must understand the problem with polygons in the early 90s. True 3D rendering involves calculating the position of vertices (points) in a 3D space (X, Y, and Z axes), connecting those points to form flat surfaces (polygons), and then painting textures onto those surfaces.

This process is math-heavy. The computer has to constantly calculate which polygons are visible to the player, which are hidden behind others (z-buffering), and how large they should appear based on distance. On the 286 and 386 processors standard at the time, rendering a complex indoor environment with textured walls, floors, and ceilings using true polygons would result in a framerate measured in seconds per frame, rather than frames per second. The hardware simply lacked the processing power (and dedicated 3D graphics cards, which wouldn't arrive for years) to do the math fast enough for an action game.

Developers were stuck. They wanted the visceral thrill of moving through a 3D space, but the hardware demanded 2D performance.

The Ray Casting Solution

Enter John Carmack, the legendary programmer at id Software. Carmack realized that to achieve the speed necessary for an action game, he couldn't render a true 3D world. He had to fake it. He utilized a technique called ray casting, a simplified, highly restrictive form of ray tracing.

Ray casting in games like Wolfenstein 3D relies on a few very strict rules about the game world:

1. The World is a Grid: The map is essentially a 2D top-down grid (like a piece of graph paper). Each square on the grid is either empty space (a floor tile) or a solid, impenetrable block (a wall).
2. Walls are Always Perpendicular: Every wall is a perfect square, intersecting the floor at exactly 90 degrees. There are no slopes, no angled walls, and no varying ceiling heights.
3. The Camera is Locked: The player’s viewpoint is fixed at a specific height. You cannot look up or down, only left and right.

Because these rules are unbreakable, the math required to render the screen becomes incredibly simple.

How It Works: Firing the Rays

Here is how the engine builds the image you see on the screen:

Imagine your computer monitor is a window into the game world. Your screen is made up of vertical columns of pixels (for example, 320 columns across in classic VGA resolution).

For every single frame of the game, the engine fires a single "ray" (an invisible line) out from the player's position, through each vertical column of the screen, into the 2D grid map.

1. The Collision: The ray travels outward until it hits a solid block on the grid (a wall).
2. The Distance Calculation: The engine calculates the distance between the player and that point of intersection.
3. The Drawing: Because the engine knows how far away the wall is, it knows how tall to draw that vertical slice of the wall on the screen. A wall that is close is drawn tall; a wall that is far away is drawn short.
4. The Texturing: The engine then takes a vertical sliver of the wall texture (like a picture of bricks) and scales it up or down to fit that height.

The engine repeats this process 320 times (once for every vertical column on the screen) for every single frame.

The Speed of the Illusion

The beauty of this system is its extreme efficiency. Unlike true 3D polygon rendering, ray casting doesn't have to calculate the geometry of the entire room, nor does it have to figure out which objects are hiding behind other objects. It only ever draws exactly what the player can see in that exact moment.

Furthermore, because it draws the screen in vertical strips, it perfectly aligns with how memory was structured on VGA graphics cards, allowing the computer to blast the image to the monitor incredibly fast.

To complete the illusion, enemies and items (like guards, dogs, and health packs) were not 3D models. They were 2D images called "sprites." The engine simply determined how far away the sprite was, scaled the 2D image accordingly, and pasted it onto the screen over the background.

The Legacy of the Fake 3D

Ray casting was the ultimate programming hack. It provided the immersive, visceral feel of moving through a three-dimensional space but required a fraction of the processing power of true 3D.

When Wolfenstein 3D launched in 1992, it was a revelation. Players had never experienced such fast, smooth action from a first-person perspective on a PC. It was a technological breakthrough that proved the viability of the PC as an action gaming platform.

The technique was quickly adopted and evolved. id Software's next game, DOOM (1993), used an advanced form of ray casting (technically binary space partitioning) that allowed for varying floor heights, angled walls, and lighting effects, creating an even more convincing illusion while retaining blistering speed.

Eventually, hardware caught up. By the late 1990s, with the release of games like Quake and the advent of dedicated 3D accelerator cards (like the 3dfx Voodoo), true polygon rendering became fast enough to take over, and the era of the ray casting illusion came to an end. However, the legacy of that brilliant workaround remains; it was the essential stepping stone that allowed the first-person shooter genre to exist long before the hardware was truly ready for it.